Typically, mobile devices have at least one means of reproducing audio to the user. This is typically a loudspeaker or a speaker, which is an electroacoustic transducer; a device which converts an electrical audio signal into a corresponding sound. A loudspeaker can be placed inside a mobile device and held near to an ear of the user. This kind of construction is typically called an earpiece or receiver. In the earpiece, the loudspeaker is normally reproducing downlink audio, for example in a hand portable call. A loudspeaker may also be intended to reproduce audio that the user will hear at a distance. This audio may be, for example, ringing signals, music, or downlink speech. This kind of loudspeaker is typically called an integrated hands-free loudspeaker. A loudspeaker is typically a small dynamic loudspeaker. A dynamic loudspeaker normally comprises a permanent magnet, which results in a stray magnetic field being present also outside the loudspeaker. A dynamic loudspeaker normally also comprises a coil situated inside a permanent magnetic field generated by the permanent magnet. This coil is normally called a voice coil. The voice coil is normally attached to a flexible diaphragm. An electrical signal can be fed to the voice coil, which results in sound waves being produced by the flexible diaphragm.
Some mobile devices with earpiece functionality additionally have to be hearing aid compatible, HAC. This means that they also have to be able to provide inductive coupling to hearing aids. In some cases, a dedicated coil is used within the mobile devices for the HAC for achieving the required inductive coupling. In some other cases, the coil inside the loudspeaker itself is sufficient and no additional coil is needed.
Another desire in the design of mobile devices is to protect the delicate components inside mobile devices from damage by the environment. This is also relevant for a loudspeaker, which is more or less in a direct contact with the air outside the body of the device. A typical hazard is produced by ferromagnetic particles, such as ferromagnetic dust, abundant for example in pockets and bags, where also keys, coins and other metal objects may be present. Such dust is attracted by the permanent magnet inside the loudspeaker, after which it accumulates on the sound-generating diaphragm and impedes the function of the loudspeaker. The result is degraded or lost audio, which cannot be restored without sending the mobile device to a repair facility.